1. Field of the Invention
The present invention relates to a camera focus detecting device which detects the focus condition of the objective lens basing on the light coming from an object to be photographed and passing through the different portions of the objective lens. In particular, the invention relates to such a focus detecting device having an optical construction including a pair of image forming lenses for respectively forming an image of the object and a photo sensor array for receiving the two images and generating signals representative of the positions of the formed images for the focus detection.
2. Description of the Prior Art
The focus detecting device having the optical construction as mentioned above has been known in the art as seen, for example, in a Japanese laid-open patent application Tokkai Sho 58-150918. The optical construction of the device is schematically shown in FIG. 1. With reference to FIG. 1, the primary image 28 of an object formed by an objective lens 2 is re-imaged or re-formed by a pair of image-forming lenses 10 and 12 to be a pair of images 30 and 32 (or 34 and 36 or 38 and 40) formed of the light passing through the different portions of the objective lens 2. The pair of image-forming lenses 10 and 12 are disposed at positions conjugate with the exit pupil of the objective lens 2 with respect to a condenser or relay lens 6. In the case of re-imaging, the distance between the re-formed images 30 and 32 (or 34 and 36 or 38 and 40) changes depending on whether the primary image is formed in front of a predetermined focal plane 22 (front focus), exactly on the predetermined focal plane 22 (in-focus) or behind the predetermined focal plane 22 (rear focus). The focus detecting device has a photo sensor array disposed on a given plane 20 in the vicinity of the re-image plane such that the photo sensor array detects the distance between the re-formed images. The detected image distance is compared with the standard image distance, i.e. the distance between the re-formed images at in-focus condition, to calculate the difference between the detected and standard distance, i.e. the amount of deviation of the image distance. From the calculated amount is determined an amount of the deviation of the focusing, i.e. the distance or interval between the predetermined focal plane and the image position where the primary image 28 of the object is formed by the objective lens 2. Although the principle of detecting the amount of the focus deviation will be explained later on, the re-formed imaged are at the positions 30 and 32 in the in-focus condition, at the positions 34 and 36 in the front focus condition and at the positions 38 and 40 in the rear focus condition. The above described focus detecting system is referred to as phase difference detection type system or shortly as phase difference type hereinafter for the convenience of description.
When the optical system for the detection of phase difference including a condenser lens 6, a pair of image-forming or re-imaging lenses 10 and 12 and photo sensor array, is incorporated or housed within the camera body (for example under the bottom of the mirror box) of a single lens reflex camera, it is convenient if the pair of re-imaging lenses are made of plastic material integrally with each other and the optical system is formed into a compact module. However, plastic material generally has a linear expansion coefficient that is three to four digits larger than that of glass material. The acrylic resin, for example, has the coefficient of expansion of 6.times.10.sup.-5 /.degree.C. Thus, if a plastic material is employed for an optical component of a camera, the expansion or contraction of the plastic with temperature is not negligible with respect to the effect on the result of a focus detection. In other words, when the pair of image forming lenses are integrally molded of plastic material, the distance between the lenses changes with the ambient temperature, i.e. the temperature in the circumstance of the camera. For example, if the distance between a pair of integrally molded lenses is 1.1 mm at a certain temperature as shown in FIG. 4, the distance will increase by a 2 .mu.m with the raise of temperature by 30.degree. C. If the amount of the change of the lens distance is converted into the amount of deviation of the image formed by the objective lens 2 from the predetermined focal plane 22, the value will be as large as 85 .mu.m. It should be noted that cameras are usually used in the ambient temperature ranging from -20.degree. C. to +40.degree. C. or more. Additionally, single lens reflex cameras are generally required to have the high accuracy of focus detection of about .+-.50 .mu.m. Accordingly, if no countermeasure or compensation is adopted for the expansion or contraction of the plastic material due to the temperature change, the focus detection is affected by temperature and will provide incorrect indication, such as to indicate front or rear focus for the case when the image of an object is formed on the predetermined focal plane, i.e. for the in-focus condition. In other words, accurate result of focus detection can not be obtained with the plastic lenses under different temperature condition, the pair of integrally molded plastic lenses could not be employed for the focus detecting device for use in a single lens reflex camera for which a high accuracy is required.